The present invention relates to a steering apparatus for a vehicle for directing steerable wheels in response to operations of a driver by means of a steering mechanism which is not mechanically connected to a steering operating means that directs the steerable wheels.
Steering of a vehicle is performed by transmitting operations of a steering operating means, such as a steering wheel, arranged inside a vehicle chamber (generally through a rotating operation of the steering wheel) to a steering mechanism arranged outside the vehicle chamber for directing steerable wheels for steering.
Various types of steering mechanisms such as ball-screw type or rack-and-pinion type are practically used for vehicles. For instance, a rack-and-pinion type steering mechanism is arranged so as to transmit sliding in longitudinal axial directions of a rack shaft which is arranged so as to extend from side to side in a front portion of a vehicle body to knuckle arms attached to both front steerable wheels via tie rods. In this mechanism, a pinion fit into a tip portion of a rotating shaft of a steering wheel (steering column) extending to outside the vehicle chamber is engaged with a rack gear formed in a middle portion of the rack shaft, and rotation of the steering wheel is converted to sliding movement of the rack shaft in a longitudinal direction, whereby steering is performed in accordance with the rotating operation of the steering wheel.
A power steering apparatus is broadly popular in recent years having a structure in which an actuator for assisting steering operation such as a hydraulic cylinder and an electric motor is provided in the middle of the steering mechanism, and this actuator is driven on the basis of a detected value for a steering force applied to the steering wheel for steering. Operation of the steering mechanism corresponding to the rotation of the steering wheel is assisted by an output of the actuator, whereby labor of the driver necessary for the steering is reduced.
However, since the steering wheel and steering mechanism are mechanically connected to each other in such conventional steering apparatuses, whether it has the power steering apparatus or not, the position at which the steering wheel is provided in the vehicle chamber is limited so that the freedom of layout for the interior of the vehicle chamber is restricted. Moreover, for realizing the connection, connecting members of large volume are required so that the vehicle cannot be made light.
In order to solve such problems, there has been proposed a separated type steering apparatus wherein a steering wheel, which is employed as the steering operating means, is provided but is not mechanically connected to the steering mechanism, and an actuator for steering is provided within the steering mechanism similar to the actuator for assisting the steering in the power steering apparatus. This actuator for steering is operated on the basis of detected values of the operating direction and the operating amount of the steering operating means, and steering force is applied to the steering mechanism, whereby steering is performed in correspondence to the operation of the steering operating means. Here, an electric motor (steering motor) is generally used as the actuator for steering for ease of control of changing steering characteristic in accordance with driving conditions.
Such a separated type steering apparatus not only presents an advantage in that it can solve the above problems but also in that it increases freedom of design in its arrangement, because it can flexibly meet changes in steering characteristics corresponding to driving conditions of the vehicle such as high/low vehicle speed, turning degree or presence/absence of acceleration/deceleration, and also enables the employment of suitable steering operating means such as levers, handgrips or pedals with no mechanical limit in correspondence between the operating amounts of the steering operating means and the steering actuator.
This apparatus has many further advantages which cannot be realized in a steering apparatus wherein the steering operating means and the steering mechanism are mechanically connected with each other. The apparatus has a concrete advantage in that it can be easily applied to automatic driving systems such as ITS (Intelligent Transport Systems) or AHS (Automated Highway Systems) which have been developed in recent years. Thus, this apparatus is focused as being useful in development of automobile techniques.
While such a separated type steering apparatus offers many advantages as above-described, it also presents a problem in that the driver cannot feel in the steering mechanism a reaction force of the road surface which is fed back from the steering mechanism in accordance with the steering, since the steering operating means is separated from the steering mechanism. In order to cope with this problem, there is conventionally provided with reaction force applying means which is attached to the steering operating means to apply a force to the steering operating means which is opposite to the operating direction and to enable steering operation, while making the driver feel as if the steering operating means and the steering mechanism were mechanically connected with each other.
Since the reaction force of the road surface varies in accordance with the running condition such as vehicle speed (high/low), size of steering angle (large/small) or road surface condition, an electric motor (reaction force motor) is generally used as the reaction force applying means similar to the actuator for steering operation, taking into account the degree of reaction force control according to running conditions.
The reaction force applying means employing the reaction force motor sends an output of the reaction force motor (reaction force torque) to the steering operating means through a suitable linkage mechanism such as a gear mechanism. Control of reaction force is performed by identifying reaction force actually applied to the steering mechanism (actual reaction force) and obtaining a target value for the reaction force torque based on this reaction force, or alternatively, after correcting this reaction force in accordance with running conditions, whereby the driving of the reaction force motor is controlled to output this target value.
In controlling the reaction force as explained above, it is a prerequisite to obtain a correct actual reaction force. Taking a rack-and-pinion type steering apparatus as an example, a reaction force sensor for detecting the actual reaction force may be structured so as to comprise a distortion gauge affixed to a tie rod connecting the knuckle arm and rack shaft, and an axial force acting on the tie rod is detected by utilizing distortion as the medium. Actual reaction force may be similarly detected by utilizing distortion of the affixed portion as the medium in other types of steering apparatuses by means of a distortion gauge affixed to a suitable member within the steering mechanism.
However, the steering mechanism is arranged in a manner so as to extend from the interior of the engine compartment to steerable wheels for steering on either side, and this position is a rather hard environment in terms of detecting distortion by means of a distortion gauge, so that the obtained value for the actual reaction force lacks reliability. If control of reaction force is performed on the basis of this detected value, it may be that the desired feeling for the steering operation cannot be obtained.
Japanese Patent Application Laid-Open No. 4-133861 (1992) discloses a method wherein a target value for a reaction force torque is determined by detecting an operating position (steering operating angle) of a steering operating means, and respective gains are multiplied to the steering operating angle, a steering operating angle speed and a steering operating angle acceleration in order to apply a reaction force which basically becomes large in accordance with the increase of the steering operating angle. Although detection of the actual reaction force which may lack reliability is not required in such a method, it presents a drawback in that variations in road surface conditions such as road surface friction (high/low) cannot be transmitted to the driver since the condition of the steering mechanism which is mechanically separated from the steering operating means cannot be reflected in the target value for the reaction force torque.
Japanese Patent Application Laid-Open No. 7-186983 (1995) discloses a method wherein a target value for a reaction force torque is obtained by calculating a slip angle of steerable wheels for steering on the basis of detected values for the steering operating angle and the vehicle speed, weighting a reaction force torque obtained from the calculated value and the detected values in accordance with the slip angle, and estimating a friction force for the steerable wheels for steering from the detected value for the vehicle speed. This method presents drawbacks in that the coincidence accuracy of the estimated values for the friction force based on the vehicle speed only and of the actual friction force is uncertain, and that it is required to maintain a plurality of databases including a table for the weighting whereby the structure for the control system is complicated. It also presents the drawback that the condition of the steering mechanism is not sufficiently reflected in obtaining a target value for the reaction force torque, similar to the method disclosed in Japanese Patent Application Laid-Open No. 4-133861 (1992).